To respond to an increase in data traffic typically found in the Internet, construction of high-speed and large-capacity optical access networks rapidly progresses. Recently, a gigabit-passive optical network (G-PON) in which a plurality of subscribers share an optical fiber cable while high-speed downstream transmission of 2.4 Gbps at the maximum is available is widely used as a high-speed optical access system for constructing an optical access network.
To achieve still higher speed transmission in the future, development of a bit-rate mixed PON system in which an optical access network that has already been constructed is utilized and at the same time high-speed transmission (for example, 10 Gbps) is available only to the subscribers who require high-speed transmission is expected from the economical point of view. For example, Japanese Laid-open Patent Publication No. H08-008954 discloses a technology that implements a bit-rate mixed PON system. According to the technology, a multi-rate burst circuit is provided to an optical network unit (ONU) for each subscriber. Thus, a plurality of bit rates can be mixed in the PON system.
In an optical access network, an optical network unit (ONU) that functions as a communication terminal apparatus located on a subscriber's side is configured to extract clock according to a waveform of a received signal and to perform various operations according to the clock. In an optical access network in which signals of a plurality of bit rates are transmitted, however, an ONU that supports only lower bit rate signals sometimes cannot correctly recognize a waveform while receiving a higher bit rate signal, and cannot normally extract clock from the received signal.